U.S. Pat. No. 3,959,953 discloses a metal detector for detecting ferrous tramp metal passing with crop material through the feed of a forage harvester to a rotating cutter head. If a tramp metal object is detected in the crop material the metal detector produces an output signal to stop the feed to the cutter head before the cutter head is damaged. The metal detector is located inside a rotatable metal feed roll and in the vicinity of other rotating feed rolls, the cutter head, and other cyclically moving ferrous machine parts. These moving parts induce an output signal (noise) from the detector even though there is no tramp metal in the crop material passing through the feed. To reduce noise, magnetic shielding has been employed and machine parts have been made of magnetically transparent (i.e. stainless steel) material. Even these measures do not eliminate all noise. Therefore, to avoid the generation of a false signal which stops the feed, it has been conventional to provide a threshold detector to which the metal detector output signal is fed. The threshold detector is set so that it produces no output signal unless the sum of the noise component and the transient detection (i.e. resulting from sensing tramp metal) component of the output signal from the metal detector exceeds the threshold value.
The threshold detector method of noise cancellation is not entirely satisfactory when used alone. The noise component of the output signal from the metal detector is not constant but varies widely in magnitude. If the threshold of the threshold detector is set too high, so as not to give a false output signal when the noise is at a peak, then a detection signal occuring when the noise is at a minimum may not cause the threshold detector to produce an output signal even though it should. On the other hand, if the threshold is set too low, the noise alone, when it is at a peak, may cause the threshold detector to produce an output signal falsely indicating the presence of tramp metal.
The threshold detector method, when used alone, suffers a further disadvantage in that it is sensitive to gradual changes in noise (such as those resulting from wear of the moving parts) and thus may require adjustment from time to time.
The present invention overcomes the problems of the threshold detection method of noise cancellation by recognizing that most of the noise induced in the metal detector occurs cyclically or periodically and thus may be cancelled or filtered out by subtracting the noise occuring at a given point in one cycle from the noise occuring at a corresponding point in the previous cycle.